[cellis]

  First, charity isn't a competition over who can sacrifice the most. 
  At the end of the day, that $500,000 helps Wikipedia 
  five thousand times more than your $100. More, probably,  
  because his donation raises the profile and will convince  
  others to donate. Second, people aren't "expected" to give 
  anything, regardless of how much money they have.

But rich people are expected to give something to society, the same way powerful people are expected to use their power for good, and smart people are expected to contribute scientific breakthroughs and engineering feats, not build throwing sheep apps. The concept of noblesse oblige is the definition of this expectation. Perhaps you meant that they aren't required to give anything. No one is required to do anything except obey the law. Perhaps people shouldn't expect the wealthy to give, but that doesn't change the fact that they do.

Furthermore, it could easily be argued that Sergey Brin benefits far more from Wikipedia's existence (perhaps 5,000 times more) than ajross, and this donation is him showing his appreciation for that benefit.

However, I do agree with you on his self-righteousness, that is uncalled for. It is quite irksome to see someone criticize anyone for volunteering a large amount of their hard earned money.